This invention relates generally to vaporizers, and more particularly, is directed to a vaporizer which produces a vaporized coating compound entrained in a gas stream, for use in depositing a thin inorganic film by pyrolytic chemical vapor deposition (CVD) onto a substrate.
The desirability of applying uniform coatings to substrates, such as flat glass, glass bottles and the like for varying the mechanical, thermal, optical, chemical resistance and/or electrical properties of the glass, has long been recognized. Such coatings are generally formed from a coating chemical, e.g. a metal compound, such as an organotin compound or the like. These coatings may be deposited using a pyrolytic CVD method. Thus, for example, where coating of a flat glass substrate is involved, as a hot, freshly formed glass ribbon travels from a flat glass forming section to an annealing section, a metal or metal oxide coating is deposited on one face thereof by at least one nozzle which directs a jet of gas containing such coating compound onto the exposed face of the glass ribbon. Reaction components and unused coating compound are removed by an exhaust duct. In the case where glass bottles are coated, the glass bottles travel through a coating hood where they are coated by the coating compound vaporized in a gas stream.
Conventional systems of the type for coating a flat glass substrate are disclosed in U.S. Pat. Nos. 4,359,493; 4,387,134; 4,524,718; 4,584,206 and 4,600,654. On the other hand, conventional systems for applying a coating compound to glass bottles are disclosed in U.S. Pat. Nos. 3,516,811; 3,684,469; 3,819,404; 3,876,410; 3,933,457 and 4,389,234.
In such systems, it is necessary first to vaporize the coating compound and entrain the same in a gas stream which carries the vaporized coating compound to the glass surface to be coated. In this regard, reference is made to the aforementioned U.S. Pat. Nos. 3,876,410; 4,387,134; and 4,600,654, all of which disclose vaporizers or evaporators in a general sense.
A vaporizer is known from U.S. Pat. Nos. 3,850,679; 3,888,649; 3,942,469; 3,970,037; and 4,359,493, all assigned to PPG Industries, Inc. As disclosed in these patents, the vaporizer includes a large cylindrical chamber which is horizontally oriented. A heater is mounted within the vaporizer chamber in a manner so as to divide the chamber into two portions, an upper one into which all incoming materials enter and a lower one from which departing vapors leave. The heater is so constructed that vapors pass through it from the entrance portion to the exit portion, and a preferred embodiment of the heater is a finned tube heat exchanger having a thermally controlled heat exchange fluid supplied to the tubes thereof. A carrier gas and the coating compound to be vaporized are introduced by a spray at the upper portion of the chamber. After contacting the heat exchanger, the coating compound is vaporized and entrained in the gas and exits through the lower portion of the chamber.
It is preferably to use a chemical vapor deposition (CVD) technique to coat the glass surfaces, since this offers advantages in uniformity and deposition rate, due to less glass cooling. With such technique, organotin compounds, such as monobutyltin trichloride, are suitable for depositing thin SnO.sub.2 films. These chemicals typically have relatively low decomposition temperatures. Organotin compounds that are significantly more volatile than monobutyltin trichloride tend to be more toxic, and therefore present exposure problems in the work place. For this additional reason, monobutyltin trichloride is preferably used as a coating compound. When a conductive SnO.sub.2 film is required, a dopant precursor, such as an organic fluoride compound, may be added to the organotin.
Laboratory tests have indicated that it is generally desirable to have a high concentration of organotin compound in the vapor stream while coating, for example, on the order of 2-15 mole %. In small scale operations, high vapor concentrations are achieved by co-feeding preheated gas and liquid coating formula into a substantially tubular vaporizer, whereby air and liquid coating formula flow concurrently, with the liquid wetting all or a fraction of the vaporizer walls. Heat is provided by conduction through the vaporizer walls from an outside source, such as an electrical resistance heater or circulating heat transfer fluid, as discussed above with the aforementioned PPG patents.
However, during scale-up of such construction, difficulties can arise. This can be explained as follows. For a given vaporizer length, the heat and mass transfer area increases in proportion to the diameter of the vaporizer. However, the hydraulic capacity, that is, the capacity at which the coating compound can be supplied is approximately proportional to the cross-sectional area of the vaporizer, or the square of the diameter. Therefore, to maintain sufficient heat and mass transfer area during scale-up, an increase in the length of the vaporizer approximately proportional to the increase in capacity is required, resulting in an increased cost of construction and operation of the vaporizer.
Therefore, even at the laboratory scale, it is necessary to operate vaporizers of this type with wall temperatures exceeding the decomposition temperature, that is, the temperature at which decomposition of the organotin compound becomes noticeable, in order to prevent the vaporizer from becoming excessively long, while at the same time, providing a high concentration of the coating compound in the gas stream. As a result, known vaporizers are inefficient where it is desired to vaporize organotin compounds having low decomposition temperatures.
Further, the vaporization temperature must be less than the decomposition temperature, that is, the temperature at which the vaporized coating compound will break up on the substrate and form solids; otherwise, the vaporizing surface will become soiled and the vaporizer plugged. The problem, however, is that the vaporization temperature approaches the decomposition temperature at high vapor concentrations.
Therefore, it is desirable to obtain high vapor concentrations while also providing a high efficiency of the vaporizer. It is important to note that high concentrations can be achieved at low surface temperature, and that the approach to equilibrium can be close.